How do I add a keyframe/inbetween without moving the timeline?

look into the topic of animation photoshop using onion skinning

photoshop onion skin animation 

Thanks for the reply JJMack. I think I know what you mean, but i'm trying to add a frame in between existing ones, without pushing anything forward. I'm not using the transform on my animation, i'm drawing in each frame. I'd like to work more efficiently with a pose to pose method, kind of the way you could in flash or Maya or something. 

I'm aware of the "split timeline at playhead" feature. But are there other options, where I can just insert/move a frame between two existing ones without changing their timing?